Various stirrers have been proposed for emulsification, dispersion, or mixing of a fluid, and today it is requested that a fluid to be processed which contains a material having a small particle diameter such as a nanoparticle is processed sufficiently well.
For example, a bead mill and a homogenizer are known as examples among many stirrers widely known.
In a bead mill, however, performance deterioration due to destruction and damage of a crystal condition of particle's surface has been a problem. Another significant problem is that a foreign matter is generated. In a high pressure homogenizer, problems relating to stable operation and requirement of a significantly large energy are yet to be solved.
A rotary homogenizer has been used as a pre-mixer in the past; but this requires a finishing machine to accomplish dispersion and emulsification to a nanometer level.
(With Regard to Patent Documents)
In view of the above situation, inventors of the present invention proposed the stirrer shown in Patent Documents 1 to 3. This stirrer is equipped with a rotor having plural blades and a screen having plural slits which is arranged around the rotor. The rotor and the screen rotate relative to each other, whereby shearing a fluid to be processed in a very narrow space formed between the blades and the inner wall of the screen which has slits so that the fluid to be processed is discharged from inside the screen toward outside thereof through the slits as an intermittent jet flow.
In the stirrer like this, as shown in the columns of Background Art of Patent Document 2, the stirring condition thereof has been changed by adjusting the rotation number of the impeller (namely the rotor). In the invention according to Patent Document 2, the proposal was made as to the stirrer in which the clearance between the edge of the impeller and the inner wall of the screen can be selected arbitrarily, whereby intending to optimize the capacity improvement in accordance with the fluid to be processed. In Patent Document 3, by increasing the frequency Z (kHz) of the intermittent jet flow above specific value, it was found that the effect to make particles finer was drastically enhanced; and based on this finding, the invention could be completed as to the stirrer that enabled to make the particles finer in the region which could not be achieved by conventional stirrers.
In all of these Patent Documents, the inventions were made by changing the clearance between the screen and the inner wall or by changing the frequency Z (kHz) of the intermittent jet flow, wherein these changes were made under a certain condition of the width of the edge of the rotor's blade in a circumferential direction and the width of the slits in a circumferential direction (specifically, under the fixed condition where the both widths are almost the same or the width of the edge of the rotor's blade is slightly larger than the width of the slits).
From the development work having been made so far by the applicant of the present invention, it is known that emulsification, dispersion, or mixing can be made by a liquid-liquid shear force in the velocity interface generated by the intermittent jet flow; and thus, it is presumed that this liquid-liquid shear force can effectively act so as to realize refinement of a fluid to be processed, especially to realize very fine dispersion and emulsification such as nano-level dispersion and emulsification; however, the action thereof is not yet fully elucidated until today.
(Historical Aspect of the Present Invention)
Inventors of the present invention tried to realize finer dispersion or emulsification by facilitating refinement of a fluid to be processed by the device shown in Patent Documents 1 to 3. Firstly, because shearing of a fluid to be processed occurs in a minute clearance between the blade and the inner wall of the screen having slits, it was presumed that to increase the number of shearing per unit time is effective to increase efficiency of the shearing; and thus, investigation was carried out from the viewpoint to increase the number of shearing per unit time.
As the means to realize this, as shown in these Patent Documents, to change the number of rotor's rotation (circumferential rotation velocity of the blade's edge portion) is known; however, under the condition of constant rotor's rotation number (circumferential rotation velocity of the blade's edge portion), it is presumed that to increase the number of the slits by narrowing the slits' width or to increase the number of the rotor's blades is effective.
However, in the case of generating the intermittent jet flow, if the slit's width is made too wide, a pressure of the fluid to be processed that goes through the slit decreases, on the other hand, if the slit's width is made too narrow, a flow amount of the fluid to be processed that goes through the slit decreases; and thus, there is a fear that the intermittent jet flow may not be generated favorably. As a result, there is a limit in the method wherein the slit's width is made narrow so as to increase the number of slit.
On the other hand, in the case of studying to increase the number of the rotor's blade, if the number of the rotor's blade is increased with keeping the blade's width unchanged, the space volume among the blades decreases, resulting in decrease of the amount of the fluid to be processed that is ejected by the blade, thereby suggesting that in order to increase the number of the blade, narrowing of the blade's width may be effective. On the contrary to the expectation, however, when the test was conducted to increase the number of the blade with narrowing the blade's width, refinement of the fluid to be processed could not be facilitated.
Therefore, we focused not on the increase of the number of shearing per unit time but on the liquid-liquid shear force due to the intermittent jet flow; and thus, facilitation of refinement of the fluid to be processed was studied by increasing this shear force.
The result of the study of the generation mechanism of the liquid-liquid shear force due to this intermittent jet flow will be explained hereunder with referring to FIG. 6. When the blade 12 rotationally moves by rotation of the rotor, the pressure of the fluid to be processed increases in the front side of the rotational direction of the blade 12. With this, the fluid to be processed is ejected as the intermittent jet flow from the slit 18 which is located in the front side of the blade 12. As a result, the liquid-liquid shear force is generated between the fluid to be processed that is present outside the screen 9 and the fluid to be processed that is ejected as the intermittent jet flow. Accordingly, by increasing the flow rate of the intermittent jet flow to be ejected, the liquid-liquid shear force can be increased, but there is a mechanical limit to increase the rotation number of the rotor.
As a result of further study, it became clear that the pressure of the fluid to be processed is decreased in the back side of the rotational direction of the blade 12 so that it causes a phenomenon that the fluid to be processed is sucked from the slit 18 which is located in the back side thereof. As a result, the inventors reached the idea that outside the screen 9, the intermittent jet flow of the fluid to be processed from the slit 18 is not merely ejected to the fluid to be processed that is in a static state, but a forward flow and a backward flow (ejection flow and suction flow) are generated to cause the relative difference in the velocities in the interface of the both flows thereby generating the liquid-liquid shear force between the fluids to be processed.
On the basis of this idea, the conventional examples shown in FIG. 6(C) and FIG. 6(D) were reexamined; and it was found that the thickness of the blade 12 was made as thin as mechanically allowable and the width of the edge portion 21 thereof was set narrow in order to increase the space between the blades 12 and so forth. Therefore, it became clear that because of this, the cycle of the change between ejection and suction becomes short thereby causing frequent change thereof, but possibly, the fluid to be processed cannot fully follow the change of the state between ejection and suction.